OK I watched the video and is he ever beautiful!!!
It is so sad- his stumbling and wing issue. He almost looked like he was trying to walk on his toes a bit to me, but I am not sure. Here is a post mentioning a blood test for mareks.
https://www.backyardchickens.com/t/709840/mareks-disease...
It says the video is private...I cannot see the video. I probably won't be able to help as I have no experience with Mareks, but wanted to see the video to try to think of other things it might be.
I have only dealt with mites, not lice, but I have read plenty of posts on BYC stating that it was only the sick one who is infested with lice.
However, if you can, I would treat everyone, as it was enough to send me into tears having mites crawling over me...I know it is awful for them too...
I don't know about his neurologic behavior, but mites/lice can kill chickens so it is best to keep them bug free.
Poultry dust from the feed store- shake it in the coop and toss all bedding/nesting material. Put it in a tied-off sock and dust under wings, vent area, everywhere but the face...